The effect of the pre-existing surface crack morphologies on the thermal fracture of ceramic coatings
Abstract
The fracture or delamination of ceramic thermal barrier coatings (TBCs) at the ceramic-bondcoat interface can be induced by the vertical surface cracks due to the high heat flux loading. Previous studies, on the other hand, indicated that a population of preexisting through-thickness surface cracks can increase the tensile in-plane compliance of the TBC layer, therefore reduce the energy driving the interface fracture and enhance the durability of the TBCs. However, in these studies, the effects of the length of the surface pre-cracks and the thickness of the coating were not considered. Therefore, the influences of the complete coating configurations including the pre-crack length, pre-crack density and the coating thickness, on the TBC surface and interface fracture, are not yet well understood. In this work, the finite element method was used to investigate the effects of surface pre-crack morphology (crack density and length), coating thickness and high heat flux loading on the delamination cracking at the TBC interface. Laser thermal shock experiments on real TBC specimens were also performed to verify the results from the analyses. The analytical model was able to predict the surface crack growth and the resulted interface crack driving force as a function of the pre-crack morphology. The results agreed with the laser tests and indicated that optimum pre-crack morphology can minimize the crack driving force at the interface, and thicker coatings also have the potential to be more resistant to the interface fracture. Based on this model, the methodology for designing the TBC pre-crack morphology to increase the coating fracture resistance was introduced. Finally, the thermal loading parameters other than the maximum surface temperature were studied. It was found that, the temperature difference through the coating thickness, the duration of the heating and the cooling schemes after the thermal loading all greatly affect the TBC thermal fracture behaviors.
Degree
Ph.D.
Advisors
Kokini, Purdue University.
Subject Area
Mechanical engineering|Materials science
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